Ducts are used for different purposes, including for the conveyance of air, such as in ventilation, heating and cooling systems, or for conveying away exhaust gas from clothes dryers or other similar machines, as well as for carrying electrical cables and wiring, or other utilities.
When used for air conditioning or ventilation systems, such as within suspended ceilings, particularly in industrial and office premises, the ducts are circular and must be supported, as they have little self-support.
A further, very well known use of ducts is an exhaust vent for clothes dryers, in which the duct is fabricated of a resilient wire helix covered with vinyl or aluminum tubing. Both type of ducts lack structural integrity, while the vinyl-covered duct is not flame resistant. The lack of structural integrity of these ducts typically results in sagging and crinking thereof, causing the duct, over time, to become lined with lint from the clothes dried in the dryer, posing a fire hazard. In the United States alone, thousands of fires associated with clothes dryers occur, causing deaths and injuries and millions of dollars in damages. It is generally recommended by clothes dryer manufacturers not to use vinyl ducts such as these for dryer exhaust transition ducts.
Representative of the prior art in ventilation systems is U.S. Pat. No. 5,281,187 to Whitney, for a “Unitary Vent and Duct Assembly” which discloses a “semi-rigid flexible duct” for a ventilation system installed with a suspended ceiling structure. The duct taught in this patent is actually a solid aluminum tube which is corrugated or “accordion-folded” so that it can be compressed or compacted for storage or shipping. The corrugated aluminum tube duct taught therein, is meant to be coupled to a duct assembly of which it is an integral part, which is intended only for installation within a framed section of a suspended or dropped ceiling. However, once such a tube has been compressed and then re-extended for installation, it is unlikely to maintain its rigidity, depending on the thickness of the aluminum. A tube of this type maintains its rigidity by virtue of its being fabricated of solid metal, is heavy and expensive and can be unwieldy to install. The corrugated aluminum, when extended after compression, has significant ridges and other obtrusive topographical features along its interior due to the corrugations, which cause frictional resistance to the air flow within, a further disadvantage.
Corrugated aluminum is also employed for the exhaust vent of clothes dryers, as, for example, in U.S. Pat. Nos. 5,121,948, 5,133,579, and 5,145,217, which solve the above-described problem of insufficient rigidity, but by using totally rigid segments. Even though the aluminum tubing itself is clearly fire resistant, the ridges and other internal topographical features similar to those mentioned hereinabove with respect to the Whitney patent, also cause frictional resistance to the air flow within, permitting accumulation of lint, which, as stated hereinabove, presents a fire hazard.
U.S. Pat. No. 5,526,849, included herein by reference, to Gray for a “Flexible Duct” discloses a duct and a method for manufacture thereof. The duct disclosed therein is formed of plastic tapes wound on a rotating mandrel with a wire resilient helix and a yarn helix therebetween. The duct so produced, while flame resistant, has rigidity limited to that provided by the wire helix. The additional yarn helix complicates the manufacturing process and adds to the internal topographical features of the duct, increasing friction and the possibility of lint accumulation therein, as described above.
The shape of ducts also has significance. Relatively heavy, rectangular metal ducts, formed of heavy gauge sheet metal, are often used for heating and cooling systems in industrial and office premises. A rectangular cross-sectional shape is desired due to the possibility of placing the duct against a support surface, and thereby utilizing available space more efficiently than a circular duct. These ducts are limited, however, in length, due to their relatively heavy weight and rigidity, as well as to transportation considerations. Accordingly, several lengths of these ducts may need to be joined together on site in order to provide adequate lengths of duct. It will be appreciated that they also require sufficiently strong hangers and other mechanical supports to be provided so as to provide adequate support. Furthermore, specially made corner portions must be provided to take account of bends.
A further consideration that must be taken into account when installing exhaust ducts for directing exhaust gases from machines, is the fact that the exhaust vents (or in the case of air conditioning units, the cool air supply vents) often have a square or rectangular shape, requiring somewhat unorthodox adaptive connections to conventional round ducts.
During manufacturing of conventional round ducts, a problem has been encountered with wrapping of semi-rigid materials, such as thin aluminum sheet, about a rotating mandrel. While the semi-rigid materials provide flexibility to the resulting duct produced in this fashion, the manufacturing process is complicated due to the fact that semi-rigid materials may tear under the tension applied during the wrapping procedure. The solution to this problem, until now, has been to avoid the use of thin aluminum sheets in constructing semi-rigid ducts, and to rely on heavier, more rigid materials, which do not lend themselves to flexibility, and are unwieldy to install, as mentioned above in relation to U.S. Pat. No. 5,281,187 to Whitney.
Therefore, it would be desirable to overcome the above-mentioned disadvantages associated with the prior art of semi-rigid flexible ducts.